(1) Field of the Invention
The present invention relates to a power transmission system and also to an aircraft having both a rotary wing and such a power transmission system.
The invention thus lies in the field of main gearboxes for rotorcraft.
(2) Description of Related Art
In particular, a rotorcraft type aircraft has a rotor that contributes at least in part to providing the aircraft with lift. A helicopter may thus have a rotor referred to as a “main” rotor that contributes to providing the helicopter with lift and with propulsion, and a rotor referred to as a “tail” rotor that contributes to controlling yaw movement of the helicopter.
An aircraft referred to as a “hybrid” aircraft for convenience may have a rotor referred to as a “main” rotor that contributes to providing it with lift and propulsion, together with at least one lateral rotor contributing to propelling the hybrid aircraft and to controlling the yaw movement of the aircraft.
In order to rotate each rotor, a rotorcraft is provided with a power plant comprising at least one engine.
In addition, a power transmission system is arranged between the engine(s) and each rotor in order to enable the rotors to be rotated. A power transmission system serves in particular to drive the rotor at a speed of rotation that is low compared with the high speed of rotation of the engine. By way of illustration, an engine of the turboshaft engine type possesses a drive shaft that rotates at a speed of the order of several tens of thousands of revolutions per minute, while the main rotor rotates at a speed that may, by way of example, lie in the range 250 revolutions per minute (rpm) to 450 rpm. By way of information, the lateral rotors of the hybrid aircraft rotate at a speed of about 2000 rpm, for example, while a tail rotor rotates at a speed of about 5000 rpm, for example.
A helicopter may be provided with at least two engines. Each engine drives a main gearbox of the power transmission system, possibly via at least one overrunning clutch or “freewheel”. Such a main gearbox is known under the acronym MGB. The main gearbox then drives the main rotor in rotation.
A power transmission system of a prior art helicopter presents one inlet mechanical drive system per engine. Each inlet mechanical drive system is thus driven by one engine. The inlet mechanical drive systems serve to drive a combining gearwheel. The combining gearwheel then drives a main gearbox. In particular, the combining gearwheel is constrained to rotate with a pinion of the main gearbox. That pinion then drives rotation of a main gearwheel of the main gearbox. The main gearwheel then drives a rotor mast via at least one epicyclic speed-reducing stage. In addition, the combining gearwheel drives the tail drivetrain.
Document U.S. Pat. No. 2,911,851 describes a main gearbox suitable for driving a main rotor and a tail drivetrain.
Document FR 2 568 541 describes an alternative architecture. The main gearbox comprises one inlet mechanical drive system per engine. Each inlet mechanical drive system is thus driven by a respective engine. The inlet mechanical drive systems drive a combining gearwheel that drives the tail drivetrain. In addition, each inlet mechanical drive system meshes with a first speed-reducing stage engaged with a main gearwheel. The main gearwheel drives a rotor mast via a second speed-reducing stage that is epicyclic.
A hybrid aircraft may be provided with at least two engines. In a known transmission system, each engine drives a pinion referred to as a “first” pinion, for convenience. Each first pinion drives an inlet mechanical drive system. Each inlet mechanical drive system is thus driven by one engine, via an inlet pinion, in particular. The inlet mechanical drive systems act jointly to set a main gearwheel into motion. The main gearwheel then drives a rotor mast via at least one epicyclic speed-reducing stage.
Furthermore, each first pinion drives a lateral drivetrain. Each lateral drivetrain may be relatively complex in order to follow a non-rectilinear path so as to reach a lateral rotor. For example, a lateral drivetrain may include angle transmission means of non-negligible weight.
The architecture of a helicopter power transmission system thus differs from the architecture of a hybrid aircraft power transmission system. As a result, the main gearbox of a helicopter is therefore different from the main gearbox of a hybrid aircraft.
To illustrate this aspect, the Internet site www.avxaircraft.com presents a page entitled “OH-58D/AVX conversion process”. Four drawings show diagrammatically a process for transforming a helicopter having a main rotor and a tail rotor into an aircraft having two contrarotating main rotors and two ducted thrusters. In that process, the power transmission system needs to be changed during the transformation, with each aircraft having its own power transmission system.
Documents FR 2 964 948, U.S. Pat. No. 6,042,499, and US 2006/269414 are also known.